Processing wastes obtained from the woodworking and pulp and paper industries is a priority problem in the recycle feedstock processing industry. Production of wood chipboards and squared bars from sawdust and tree bark helps prevent excessive tree felling and thus improve the plant's ecological situation. Furthermore, the processing of domestic wastes accumulating around cities has a favorable effect on air purity and sanitary conditions of the environment.
However, the existing industrial methods for processing wastes of the woodworking industry use polymeric resins which are toxic to humans and animals as binding agents. The problem is, therefore, that of dispensing with toxic binding agents in the manufacture of chipboards and bars.
Equipment used for carrying out the existing molding methods has, as a general rule, a very complicated design because of the need to apply great molding pressures.
Known in the art is a method for extruding plastic materials to produce elongate articles (see for example, U.S. Pat. No. 3,874,207, 1975). In the prior art method, the material is forced through a die by applying one or more kinds of energy, for example, heat energy, ultrasonic oscillation energy, and electromagnetic energy.
The apparatus used to carry out the above method for extruding plastic materials comprises an extrusion chamber which encloses a piston for propelling the extrudable material which is formed into a product and a die. The apparatus further comprises means for imparting oscillatory movement to the piston, which means are activated when a required amount of plastic material is accumulated between the piston and the die in the extrusion chamber. As the means are activated the piston performs a series of oscillations and pushes the material through the die to shape it into an elongated product. Following this, the outlet is closed, and a new batch of material starts to be accumulated in the extrusion chamber.
Said means for imparting oscillatory movement to the piston can alternatively be an induction coil that generates a variable electromagnetic field cooperating with the piston, or another device for example, the means comprises a nozzle to supply hot gas into the chamber at the back of the piston, and an ignition plug. A certain amount of gas is accumulated in the chamber at the back of the piston and is ignited by a spark. The pressure of the expanding gases forces the piston to move toward the die, thereby extruding the plastic material. Under the effect of residual gas pressure, the piston performs several oscillatory moves toward the die.
Said apparatus is used for extruding plastic materials. It can only be used for molding elongate products out of plastic materials when one portion adheres to the other and is held in place by the cohesive forces between the molecules of the plastic material, whereupon the material is polymerized to turn into a continuous and integral product.
The above method is unsuitable for molding mixtures comprising a plastic material as a binder and solid filler particles if the latter have sufficiently large dimensions, in which case no cohesion occurs between the solids at the molecular level. Then, after the molded product has left the extrusion chamber, and before the binder has polymerized, the product is likely to disintegrate, i.e., the bond between the individual portions of the extrudable mixture may be broken.
Apparatuses comprising units to prevent disintegration are used for molding mixtures of this type.
Known in the art is a method for molding shaped products, comprising mixing a filler, containing specially treated particles of vegetable origin, which are disintegrated and sorted according to fractions and of which one fraction is used, and a binder. The resultant mixture is fed to a briquetting zone to form a briquette with an axial passageway, whereupon the briquette is packed and advanced to a molding zone, where a continuous shaped blank is produced by joining each successive packed briquette to a preceeding one under pressure. The blank is then heated, and products of desired length are detached from the continuous shaped blank thus produced (see for example, German Patent No. 2,932,406, 1981).
An apparatus for carrying out the above method comprising as a charging and briquetting chamber, housing a hole-making member, the geometric axis of which is aligned with the molding axis of a blank, and a piston displaceable along the molding axis, the chamber and the piston being mounted on a frame and communicating with each other. The charging and briquetting chamber communicates with a briquette molding chamber which molds the briquettes into a continuous shaped blank, and a heat treatment chamber provided with a heater. The apparatus further comprises means for dividing the blank into products of desired length (see for example, German Patent No. 2,932,406 A1, 1981).
The above apparatus is suitable for molding a mixture comprising vegetable particles and a binder. A synthetic polymeric material is used as the binder. To mold the mixture, the polymeric material is first transferred to a flowable state. For one portion of the molded mass to better engage another, the piston endface is designed so that its middle areas are offset backward relative to the peripheral areas, i.e., a recess is formed. The recess has a waving, rather than a smooth,surface, the wave size descreasing toward the center. To prevent the breakup of the continuous blank, the apparatus has a cooled and a heated portion in the blank passageway. Furthermore, a gap is provided between the two passageway portions to provide heat insulation between them.
To prevent the breakup the passageway is diverging along the path of the blank, the divergence being different for different portions.
The above-mentioned method for manufacturing a continuous shaped blank uses an organic binder which is by itself toxic to humans and animals. The preferred filler is provided by the conifer chips which are easily formed into a mat. To achieve the desired strength of the final product, the above-mentioned patent provides for attaining an intensive matting state of particles of vegetable origin. Tree bark cannot, in practical terms, be used as a filler; if it has to be used, it must be disintegrated minutely. Particles of reduced size, however, require growing amounts of the binder. To reduce the consumption of the binder, this chip size increases, thereby adsversely affecting the strength of the final products.
The mixture is fed into the charging and briquetting chamber by a triple reciprocation of a gate valve. This procedure is required to remove the vapor-gas mixture and toxic gases released from the polymeric binder.
A product manufactured by this method has a density of 0.35 to 0.85 g/cm.sup.3. Moreover, the final product is easily flammable and burns well, because it is manufactured on the basis of an organic polymeric binder; also, it swells readily and breaks up under the effect of moisture.